Horizon sensors are used on orbiting bodies such as spacecraft and satellites to provide attitude information with respect to two orthogonal axes representing pitch and roll. The sensors may provide for scanning across the disk of the earth to provide at least two horizon crossings or points of optical radiation discontinuity which are viewed by a detector, e.g., infrared detector mounted in the sensor. To avoid scanning, a radiation balance type sensor has been used in which detectors in the sensor view radiation from areas extending across the horizon at several points which are then employed in an algorithm to compute pitch and roll. In the past, the assumption of the earth radiance model in the CO.sup.2 band has been a north south seasonal radiance variation. Note the performance of both scanning and starring, static earth sensors are affected by radiance variation at the earth's limb.
A typical earth sensor operating at geosynchronous altitude might have four fields viewing the horizon at different points, such as A, B, C & D in FIG. 1. The shaded area of each of these rectangular fields is the portion receiving earth radiation and producing output signals V.sub.A, V.sub.B, V.sub.C & V.sub.D. The roll axis of the spacecraft is assumed parallel to the equator and the pitch axis parallel to the polar axis. If the earth radiance is uniform, pitch and roll can be derived simply as follows: EQU Pitch P=V.sub.A -V.sub.B =V.sub.C -V.sub.D EQU Roll R=V.sub.A -V.sub.C =V.sub.B -V.sub.D
The two independent values obtained for both pitch and roll can be averaged to reduce any error by 1/.sqroot.2.
In actuality, there are large variations in radiance with latitude and season. To correct for this, compensation detectors A.sub.o, B.sub.o, C.sub.o & D.sub.o are frequently added as shown in FIG. 1. These detectors measure the radiance in the vicinity of the position sensing detectors and apply a correction to each, in accordance with U.S. Pat. No. 3,486,024. However, this assumes that the radiance is uniform over the "position sensing" and "compensation" detectors. This is not so in the winter hemisphere where significant longitudinal radiance variations occur caused by storms, extra tropical cyclones, and frontal systems. These produce radiance differences between the position sensing field and its compensation field and can cause substantial pitch and roll errors.